Lipidomic approaches now gaining acceptance are based on the rapid, comprehensive mass spectrometry (MS) analysis of carbon length and degree of unsaturation in fatty acyl groups and their positions (regiospecificity) at the sn 1,2, or 3 positions of glycerolipids. The most important analytical issue facing lipidomic analysis is that no general method has emerged that delivers double bond position in the fatty acyl groups simultaneously with regiospecific information, We propose to exploit a novel mass spectrometric method developed in our laboratory that should enable complete, automated characterization of all acylated lipids in crude lipid extracts, Ion-molecule reactions that result in adducts during chemical ionization (Cl) mass spectrometry (MS) are effectively a gas phase derivatization process. In a series of 8 papers since 1999, we reported co-discovery of gas derivatization of unsaturated fatty acid methyl esters (FAME) by acetonitrile CI/MS/MS. Fragmentation of the resulting adducts provides the first rapid, convenient structural analysis of FAME. Acetonitrile CI produces an ion at m/z 54 which adds covalently across double bonds; the resulting [M+54]+ ion undergoes charge-driven fragmentation upon collisional activation to yield diagnostic ions that unequivocally establish the position of double bonds in polyunsaturates. Recently, this reaction has been observed for atmospheric pressure CI in a liquid stream. We propose a two pronged approach where we 1) develop APCI based on acetonitrile for analysis of double bond position for fatty acyl groups within lipid classes derived from crude lipid extracts, and 2) complete development of acetonitrile GClCIIMSlMS to include analysis of all polyunsaturated non-methylene-interrupted fatty acids. These goals will eventually be coupled, and together with conventional MS/MS modes (precursor, product, and neutral loss scans), analysis of gas phase derivatized acyl-containing compounds should provide complete structural and quantitative information for most glycerolipids and can be automated for lipidomic analysis.